Tetrafluoroberrylate(2-) ions as hydrogen-bond proton acceptors: Quantum chemical considerations

The experimentally observed lower proton acceptor ability of the tetrafluoroberrylate(2-) anion, compared to that of the sulphate(2-) and selenate(2-) anions, was explained on the basis of various quantum chemical approaches. The geometries of the above-mentioned XY42- anions were fully optimised at the ab initio HF SCF and MP2 levels of theory, as well as within the DFT (BLYP and B3LYP) approach, using the 6-311+G* and LANL2DZ basis sets. The 6-311++G(3df,3pd) basis set was also used within the DFT BLYP approach. The molecular charge distribution was partitioned among the atomic centers using the Mulliken as well as the natural population analysis. All levels of theory (both with and without inclusion of the electron correlation effects), regardless of the basis set employed, predict lower (by absolute value) atomic charge on the fluorine atom in the tetrafluoroberrylate(2-) anion than that on oxygen atoms in sulphate(2-) and selenate(2-) anions. The quantum chemical predictions are fully in line with the spectroscopically observed order of the proton acceptor abilities in these anions.